The invention relates to a method for operating an injection system for injection molding machines with a melting cylinder with melting worm and two electric motors, preferably servomotors, for driving the melting worm whose rear end is connected so as to be torsion-tight with a spindle, which can be driven by the first electric motor and on which a nut is borne drivable by the second electric motor, wherein for the injection a translational movement of the worm results from the nut being driven by means of the second electric motor.
An injection system of this type is already disclosed in DE-OS 43 44 335. The conventional operation of such injection system proceeds as follows: for melting and transporting the material into the antechamber of the worm the spindle and the nut are driven by the first and the second electric motor in the same direction. The plastifying performance results therein from the rotational speed of the worm which is determined by the rotational speed of the spindle or, respectively, of the first electric motor. In order to bring about during plastification a return-stroke speed of the worm through which the back dynamic pressure of the plastificate in the antechamber of the worm is determined, the spindle and the nut are driven at a slightly different rotational speed. For the subsequent injection of the plastificate the first motor, which drives the spindle, is stopped and the nut is driven in the corresponding direction via the second motor.
In the course of the machine cycle, the driving unit for the injection is switched between different operating states. For example, the injection speed at the beginning of the injection process increases from zero to a given value. Further switching takes place between the pre-injection and the application of the dwell pressure.
It should be possible to complete such switching processes as rapidly as possible, i.e. the dynamics of the driving system for the injection should be as high as possible. In the conventional operating type of an injection system cited above, the dynamics of the driving system for the injection through the applicable torque of the second electric motor used in combination with the mass inertia of the system is limited.
It is furthermore desirable that a maximum injection speed which is as high as possible can be brought about. In the conventional operating mode of the above cited injection system the injection speed is determined by the maximum speed at which the nut can be driven by the second electric motor.